An oil-filled type differential pressure sensor is for example known as a pressure sensor. The pressure sensor has a case, a sensing portion, and diaphragms. One of the diaphragms is provided on a first side of the case to define a first pressure detection chamber. The remaining diaphragm is provided on a second side of the case to define a second pressure detection chamber. The first and second pressure detection chambers are filled with a pressure medium. The sensing portion is provided on the case. The sensing portion outputs a signal in response to an applied pressure. This kind of pressure sensor is for example disclosed in Japanese Patent Publication No. 2003-315193 (U.S. Pat. No. 6,813,953 B2).
In the above pressure sensor, the sensing portion receives a pressure through the pressure medium of the first pressure detection chamber and a pressure through the pressure medium of the second pressure detection chamber. The pressure sensor detects a pressure based on a differential pressure between the pressure mediums of two chambers.
The pressure sensor is for example used to a diesel particulate filter (DPF) of a diesel engine for detecting a pressure loss before and after a filter. Generally, the DPF is a system for restricting soot and smoke from releasing into the atmosphere by the filter. The filter is provided in an exhaust pipe to trap the soot. When a certain amount of soot is trapped, it is burned.
As shown in FIG. 8, a case 10 of the pressure sensor forms a first recessed portion 11a on a first side and a second recessed portion 11b on a second side. A first diaphragm 81 is arranged to cover the first recessed portion 11a through an O-ring 90 so that a first pressure detection chamber is formed. Likewise, a second diaphragm 82 is arranged to cover the second recessed portion 11b through the O-ring 90 so that a second pressure detection chamber is formed.
The first and second diaphragms 81, 82 are for example made of metal. The first and second pressure detection chambers 81, 82 are filled with oil 70 as the pressure medium.
The case 10 is formed with a communication hole 11c between the first pressure detection chamber 11a and the second pressure detection chamber 11b. A sensing portion 20 is provided in the first pressure detection chamber 11a such that the communication hole 11c of the case 10 is covered. The sensing portion 20 outputs an electric signal in accordance with a degree of pressure applied thereto.
Also, a first pressure port member 12 and a second pressure port member 13 are connected to the case 10 with bolts 60 and nuts 61, 62 in a condition forced toward the O-rings 90 through the first and second diaphragms 81, 82, respectively. The ends of the diaphragms 81, 82 are adhered to the first and second pressure port members 12, 13, and located between the first and second pressure port members 12, 13 and the case 10. As such, the first and second pressure detection chambers 11a, 11b filled with the oil 70 are sealed.
For example, the first pressure port member 12 permits an entry of an upstream pressure of the DPF through an inlet port 12a. The second pressure port member 13 permits an entry of a downstream pressure of the DPF through an inlet port 13a. The pressures entering from the inlet ports 12a, 13a are transmitted to the first and second diaphragms 81, 82.
Further, the pressures applied to the first and second diaphragms 81, 82 are transmitted to the sensing portion 20 through the oil 70. The sensing portion 20 detects a pressure based on a differential pressure of the oil 70 between the first and second pressure detection chambers 81, 82.
The above pressure sensor is for example produced in a following manner. First, the case 10 is formed. In the case 10, terminals 10a and nuts 61 are embedded such as by insert-molding. Next, the sensing portion 20 integrated with a seating 30 is adhered to the first recessed portion 11a. The sensing portion 20 is wired to the terminals 10a through bonding wires 40.
Then, the O-rings 90 are placed on the case 10. This case 10 and the first pressure port member 12 to which the first diaphragm 81 has been adhered are housed in a vacuum chamber. In the vacuum, the oil 70 is injected in the first recessed portion 11a. 
Then, the first pressure port member 12 is joined to the first side of the case 10 in the vacuum so that the first diaphragm 81 covers the first recessed portion 11a in which the oil 70 has been injected. The above assembling process is performed in the vacuum to avoid entry of air into the first pressure detection chamber 11a. 
The connected case 10 and the first pressure port member 12 are removed from the vacuum chamber into the atmosphere. The case 10 and the first pressure port member 12 are fixed by fastening the screws 60 with the nuts 61, thereby to seal the first pressure detection chamber 11a. The second pressure port member 13 is connected to the case 10 in the similar manner.
In the above pressure sensor, the first and second pressure port members 12, 13 to which the diaphragms 81, 82 have been integrated are fixed to the case 10 after the oil 70 is injected separately into the first and second recessed portion 11a, 11b. Therefore, it is necessary to fix the first and second pressure port members 12, 13 in the vacuum. As such, it is likely to take time and effort to inject the oil 70 and fix the diaphragms 81, 82 to the case 10.
Also, the oil 70 is injected to each of the pressure detection chambers 11a, 11b, and the diaphragms 81, 82 are fixed to respective sides of the case 10 after the oil injection. Therefore, the amount of oil 70 is likely to be uneven between the two chambers 11a, 11b. Also, forces applied to the oil 70 by the first and second diaphragms 81, 82 are likely to be uneven between the two chambers 11a, 11b. As a result, oil inside pressure is likely to be uneven between the two chambers 11a, 11b. 